The present invention relates generally to protective coatings for plastics such as polycarbonate, and more particularly to coatings which protect the underlying substrate from damage caused by exposure to ultraviolet radiation.
U.S. Pat. No. 4,275,118 to Baney et al discloses a coating composition comprising an acidic dispersion of colloidal titania, colloidal silica and hydroxylated silsesquioxane in an alcohol-water medium which produces a hard, abrasion-resistant coating when cured on a plastic surface such as polycarbonate.
U.S. Pat. No. 4,390,373 and 4,442,168 to White et al disclose a cured transparent, abrasion resistant coating composition comprising an effective abrasion resisting amount of colloidal dispersion containing colloidal antimony oxide and colloidal silica in a weight rate of less than or equal to about 30:70 in a water-alcohol solution of the partial condensate of R(Si(OH)3) wherein R is an organic radical.
U.S. Pat. No. 4,405,679 to Fujioka et al discloses a coated shaped article of a polycarbonate type resin of improved abrasion resistance comprising a shaped polycarbonate substrate, an undercoat applied and cured on the substrate, and an overcoat applied and cured on the undercoat comprising a hydrolyzate of an epoxy-containing silicon compound, at least one member of the group of hydrolyzates or organic silicon compounds, colloidal silica and organic titania compounds, and a curing catalyst.
U.S. Pat. No. 4,477,499 to Doin et al discloses ultraviolet radiation resistant silicone resin coatings having improved thermoformability and shortened required aging achieved by the addition of a Lewis acid compound to the coating composition.
U.S. Pat. Nos. 4,500,669 and 4,571,365 to Ashlock et al disclose transparent, abrasion resistant coating compositions comprising a colloidal dispersion of a water insoluble dispersant in a water-alcohol solution of the partial condensate of silanol wherein the dispersant comprises metals, alloys, salts, oxides and hydroxides thereof.
European Patent Application No. 851102939 published 05.02.86 entitled xe2x80x9cCarbon-Containing Monolithic Glasses Prepared by a Sol-Gel Processxe2x80x9d by Baney et al of Dow Corning Corporation discloses a valuable intermediate which comprises a dispersion of a colloidal metal oxide in a water-alcohol solution of the partial condensate of a silanol having the formula RSi(OH)3, wherein the metal oxide is ZrO2, SnO2, ZrSiO4, B2O3 or La2O3.
Optical quality abrasion resistant coated plastic materials generally require a coating that protects the substrate from the damaging effects of ultraviolet (UV) radiation. Protection from ultraviolet radiation is especially important for polycarbonate, since hydrolytic degradation is apparently accelerated by UV exposure. Conventional UV stabilizers do not impart sufficient protective capacity to abrasion resistant coatings, as sufficient amounts of most typical organic UV absorbers cannot be added to abrasion resistant coatings without adversely affecting hardness and adhesion of the coating. Moreover, typical UV absorbers may gradually become deactivated after prolonged exposure, and also may gradually be leached from the composition.
In Physics of Thin Films, Vol. 5, inxe2x80x9cOxide Layers Deposited From Organic Solutionsxe2x80x9d, Schroeder notes that titanium, cerium, antimony, and lead oxides deposited from organic solutions exhibit a steep rise of absorption in the near ultraviolet range of the radiation spectrum, and in the Journal of Applied Polyiner Science, Vol. 26, inxe2x80x9cInhibition of Photoinitiated Degradation of Polycarbonate by Cerium (III) Overcoating,xe2x80x9d Klein et al disclose coating a polycarbonate substrate with cerous chloride (CeCl3)/poly(vinyl alcohol) complex.
The present invention provides protection for underlying plastic substrates such as polycarbonate from damaging ultraviolet radiation by means of transparent coatings containing cerium oxide, a stable strongly ultraviolet absorbing species. The transparent cerium oxide containing UV protective coatings of the present invention are formed from aqueous sols containing colloidal cerium oxide in additional to alkoxides of silicon and/or other metals which hydrolyze and polymerize by condensation to form a film in which the cerium is incorporated in the oxide network of the coating.
Coatings that protect plastic substrates such as polycarbonate from damage caused by ultraviolet radiation are formed from aqueous sols containing cerium oxide and a network-forming metal alkoxide such as alkoxysilanes and other metal alkoxides. The alkoxysilane may be an organoalkoxysilane, such as an alkylalkoxysilane or organofunctional alkoxysilane. The alkoxide may contain alkyl or aryl groups and may be in dimer or higher condensed form so long as hydrolyzable alkoxide groups remain. The cerium oxide is preferably prereacted with a tetraalkylsilicate, preferably tetraethylorthosilicate, TEOS. Hydrolysis and condensation polymerization occur in situ. Alternatively, the alkoxide may be partially or fully hydrolyzed, and condensed to some extent prior to combination with the cerium oxide sol.
The resulting silane/ceria sol may be used as a coating composition for either a primer or protective overcoat on a substrate or coated substrate. Or, the silane/ceria sol may be added to other coating compositions to increase their resistance to ultraviolet radiation. In either case, the coating compositions may be applied by any conventional technique, such as spraying, dipping or flow coating. The composition dries and cures to form a uniform durable coating with good adhesion to plastic substrates such as polycarbonate. The coating protects the substrate from ultraviolet radiation by providing a strong, broad absorption band in the 240 to 280 nanometer range.
In a preferred embodiment of the present invention, an alkoxide is partially hydrolyzed before adding an aqueous sol of the reaction product of tetraalkylsilicate and cerium oxide. Preferably, the alkoxide is an alkoxide of the general formula RxM(ORxe2x80x2)zxe2x88x92x where R is an organic radical, M is selected from the group consisting of silicon, aluminum, titanium, zirconium and mixtures thereof, Rxe2x80x2 is selected from the group consisting of low molecular weight alkyl radicals, z is the valence of M, and x is less than z and may be zero except when M is silicon. The organic radical of R is preferably alkyl, vinyl, methoxyethyl, phenyl, xcex3-glycidoxypropyl, or xcex3-methacryloxypropyl. The alkoxide hydrolyzes according to the general reaction for preferred organoalkoxysilanes
RxSi(ORxe2x80x2)4xe2x88x92x+yH2Oxe2x86x92RxSi(ORxe2x80x2)4xe2x88x92xxe2x88x92y(OH)y+yRxe2x80x2OH.
Condensation of the hydrolyzed alkoxide proceeds according to the general reactions 
Further hydrolysis and condensation follow.
The pH and/or degree of condensation of the alkoxides may be adjusted, if necessary, to prevent haze or precipitation upon mixing with the ceria sol. The cerium oxide sol preferably comprises about 10 to 30 percent by weight colloidal cerium oxide in water, with the colloidal cerium oxide particle size sufficiently small to minimize scattering of visible light, preferably less than 30 to 40 nanometers, most preferably less than 10 nanometers. Alkoxides of titanium, aluminum and/or zirconium may also be included in compositions of the present invention, as well as colloidal silica for abrasion resistance. Suitable silica particles have an average particle size of 10 to 20 nanometers. The ultraviolet radiation protection provided by the cerium oxide containing coating of the present invention may be determined by measuring the UV absorbance spectrum of the coating applied on a quartz substrate.